Inorganic coating composition



United States Patent "ce 3,130,061 INORGANIC COATING COMPOSITION WalterM. McMahon, La Habra Heights, and Charles G.

Abba, Bell, Calif., assignors to American Pipe and Construction (10.,South Gate, Calih, a corporation of Delaware No Drawing. Filed Feb. 6,1961, Ser. No. 87,112 16 Claims. (Cl. 106-84) This invention relates toinorganic, water-insoluble, coating compositions adapted by self-curingfor the protection of metal surfaces, especially ferrous metal surfaces,and more particularly to self-curing coating compositions composed oflithium hydroxide and colloidal silica in critical proportions as a biner and metal-protectmg metal pigment such as zinc ggst, powdered lead orflake aluminum to provide anti-corrosion protection for the metalsurface.

Coating compositions of the invention comprise lithium hydroxide andcolloidal silica binder in proportions of l lithium hydroxide expressedas lithium oxide to silica in,

M 1:40 as a minimum u to a molar ratio of 1:2.5

ese limited amounts of lithium hydroxide and colloidal silica which makeup the binder of the invention provide in the presence of metal pigment,highly desirable coating compositions of the invention which whencompared with compositions in which the lithium hydroxide ingredient isabsent are superior in significant respects and provide improvedadhesion to the metal substrate, superior working qualities with solidpigment and other components in the composition, and permit very muchthinner uniform coatings to be applied to completely cover metalsurfaces to be protected (down to about 3 mils thickness as dry film).Surprisingly, the compositions of the invention exhibit improved filmflexibility in such thin coatings while providing improved hardness andimproved self-curing characteristics which is apparently based uponchemical interaction of the lithium hydroxide component with the finelydivided metal pigment and silica present as essential ingredients in thecoating composition.

The achievement of these superior coating characteristics mentioned inthe preceding paragraph is based upon adjustment of the proportions ofbinder components, lithium hydroxide and colloidal silica within theabove stated molar proportions.

If the amount of lithium hydroxide component is reduced to such a levelas to provide a mol ratio of Li O/SiO- which is less than about 1:40,for example in the range of about 1:100 molar ratio, the physical andchemical properties of enhanced adhesion, improved fiexibility, betterworking characteristics and enhanced selfcuring characteristics are notobtained. Indeed, the resulting coating compositions containing lithiumhydroxide below the minimum value are comparble with those in whichlithium hydroxide is absent.

Coating compositions containing colloidal silica binder which issubstantially free from the effective minimum amount of lithiumhydroxide as used in the invention may be satisfactory in certainrespects, but these silica binder compositions do not exhibit theimproved physical and chemical properties and advantages of the lithiumhydroxide containing coatings of the invention, particularly theself-curing characteristic. It appears that a synergistic improvement ofproperties is due to the presence of lithium hydroxide which interactsin a new manner with finely divided metal pigment to provide self-curingproperties and outstanding resistance of the wholly inorganic coating.

If the amount of lithium hydroxide binder in the metal pigmentcontaining composition of the invention is more Patented A r. 21, 1964than will provide a mol ratio in respect to colloidal silica of 1:25,for example if a mol ratio value of LI20/Si02 of about 1:2 is used, thenthe chemical interaction of lithium hydroxide with colloidal silicatends to make the 5 composition unstable under the conditions ofalkalinity existing in the liquid aqueous inorganic coating composition,there is a tendency toward the formation of lithium disilicate which isa relatively insoluble compound. Accordingly, precipitation and gellingof the liquid coating composition occurs on standing and thereby makesthe composition unsatisfactory for commercial application. At a moleratio Li O/SiO of 1:1, which is reached when still more lithiumhydroxide is added, the alkalinity increases and lithium metasilicate isformed. Lithium metasilicate, like lithium disilicate tends to r cilaud,

that the preseggggflothofthesest Qfilfijiillnsatisfactory in the coatincO QQQSiliQmeresulting in bar'war'iitng qualities and in practice,preventing uniform coating application.

Within the specified upper and lower limits of mol ratio Li O/SiO from1:40 up to 112.5 in accordance with the invention, preferred coatingcompositions can be formulated within these limits for specific metalpigments to provide outstanding properties of film ad 'ness, filmflexibility, self-curing properties characteristics. In formulating thebinder of te inven-, tion with finely divided zinc, the molar ratio ofLIzO/SIOZ lies within the more limited range of molar ratio 1:20 up to1:3.

0 Lithium silicate in colloidal form although unsuitable per se as abinder due tg it s,low.solnbilityein.-.neutra-l-- gneous solution asabove mentioned, nevertheless hy drolyzes slowly to form lithiumhydroxide and silica. By

suitable adjustment of alkalinity in the form of NaOH for example,lithium silicate is obtained from the source of lithium hydroxide andalkalized-stabilized colloidal silica in a form usable as the binder ofthe coating composition of the invention.

The invention, therefore, includes methods of prep 40 ation of lithiumoxide and colloidal silica binders in the proportions mentioned abovefor use with metal pigments by adding small amounts of lithium hydroxideto a colloidal silica sol in the presence of alkali, preferably so 1 mhydroxi r potassium hydroxide, which serves as stabilizer for thelithium oxide and silica components and provides a colloidal mixture ofbinder ingredients functioning in combination with the finely dividedmetal protective pigment for improved In one method of the inventionthere are mixglaqueous l it l 1 ium h yd rgxide solution and colloidalsilica sol irTwater at about 20-50% solids in the presence of alkalimetal hydroxide stabilizer such as sodium hydroxide or potassiumhydroxide to form the alkaline aqueous, inorganic binder, the amounts ofeach of the binder ingredients adjusted within the stated critical molarratios gLi g/sio, 1:40 to to 1:25 and the finely divi meta pigment iswell mixed wi the aqueous alkaline inorganic coating vehicle.

Another method of the invention comprises starting from aqueous collo'soluti in which WM the mol ratio of Li O/SiO lies at about at least 1:25

and adjusting the mol ratio of Li O/SiO if needed by stbilized (sodium hdroxid viding the binder in more stable colloidal form, this binderbeing mixed with metal pigment as stated in the previous paragraph.

The protective metal pigments used in the invention such as zinc dustpowdered lead or aluminum flake are preferably less than 15 microns inparticle size. Im-

0 proved fiexi 1 ity o e resulting coating may be realized by employinga mixture of a major proportion (60-90% by weight) of finely dividedmetal pigment of a fine LRIUVHNLH particle size, for example zinc ofuniform particle size f. well as thicker films, e.g., up to 2-4 mils inthickness on the metal base. For very thin coatings, e.g., about 0.5-1.0mil in film thickness, there may be used a 50/50 miweightflofluery fineparticles of meial, su ch as I zinc, in the range of 1-3 microns aii'drelatively'coarser particles of zinc in the range of 8-12 microns.

If there is used substantial amounts of particles of metal above 20micron size, more than 20-25% by weight, then there is a tendency forthe film to crack on rymg. i If the particle size range of theprotective metal such is zinc lies within a very narrow range, e.g., 95%I2-3 icrons, the films which are produced are not completelysatisfactory in every respect for flexibility, adhesion and ortions ofparticles of metal pigment be in the larger size ranges, e.g., 6-12microns and the remaining Fhe proportion of finely divided metal pigmentof preferred screen size which is employed with the lithiumhydroxide-colloidal silica binder of the invention may vary from about1:1 weight ratio up to about 25:1 weight ratio on a dry wegiht basis ofmetal pigment to hinder, depending upon the specific gravity of thepigment, and also upon the hydrophilic or hydrophobic surfacecharacteristics of the metal binder. Using zinc dgst, which is a mediumspecific gravity pigment and which is preferred for many applications toprovide corrosion protection for the underlying metal structure,especially ferrous metals, there are used from about 8 to about 20 partsof finely divided zinc metal to binder on a dry At less than about 8parts of zinc per part of binder, there is a tendency to spotting, e.g.,the formation of light areas in the film, indicating an insufiicientmetal pigment for properly covering and protecting the underlying metal.

On the other hand, lead dust which is a heavier pigment than zinc dustmay be used in a higher weight ratio of binder, the ratio being up to 25to 1 by weight. Aluminum flake, aluminum dust, magnesium dust ormagnesium flake, which are all examples of much lighter metals than zincor lead may be used in proportions to hinder (dry weight basis) as lowas 1:2. In fact, aluminum or magnesium in flake form have such excellentcovering power in comparison to zinc bases upon their leafingcharacteristics that the preferred upper limit of dry weight ratio metalpigment to binder is about 1:3. In most cases usingaluminum or magnesiumdust proportions lying between 1:1 and 1:2 on a Weight basis arepreferred for most coating uses.

Finely divided insoluble inorganic compounds as further additives may bedesirably incorporated with the metal pigment to serve as pigments, asextenders and as auxiliary anti-corrosion agents. The inorganic extendercomp u e r 51, lead di e, ea c oma e, zinc gxide, zinc chromateal'fiiliix dignd aluminum oxide.

a titanium dioxide The piments in lithopone', Zll'lC su e e, caron ac,utraclude iron oxid 1 used in the coating composition generally possessa speparticles be in the smaller size ranges, e.g., l-4 microns. J

marine blue, Hansa yellow, sienna, burnt umber, china 7 clay, blancfixe, chromium oxide, and chrome yellow among the commonly availablepigments.

Flexibilizing extenders. such\as mica, bentonite and xfoliatedvermiculite may also be 5713? The foregoing metal compound extenders andpigments matter of minutes.

cific gravity which is lower than that of zinc metal pigments and theseextenders are usually used on a dry weight ratio relative to binder ofabout 2:1 to 5:], the heavier pigments being employed at the higher endof the range.

An advantage of the lithium hydroxide-colloidal silica binder of theinvention lies in the greater compatibility of this binder for diverseinorganic pigments such as listed above which pigments inherently varyin their acidity or alkalinity from acid to alkaline pH. With otherbinders, these pigments frequently cannot be used zecause they arereactive. Yet with the binder of the inention all but the most acidpigments can be used withjout substantially interfering with the desiredcathodic protection which is provided by the metal protecting pigment.Rather wide rangesmflinert filler may be added. 1 However, it ispreferred to use up to a out 15% of inert filler by weight of metalpigment. This amount does not uring power. Hence, it is desirable thatsubstantial prodegrade the essential corrosion inhibitingcharacteristics of the coating and contributes to desired economy ofingredients for formulation.

The formulation of the coating composition containing the essentiallithium hydroxide-colloidal silica film forming binder and metal pigmentmay be varied widely to achieve any desired color. Even if lesseramounts of metal pigment are used, auxiliary metal compound as extenderand anti-corrosion additive can be added to reduce the cost. Theaddition of cheaper pigments and extenders permits economies withcertain colors at the same time, making no sacrifice in the self-curingcharacteristics based upon the binder containing metal pigment and theexcellent physical and chemical properties of the film.

In the above stated proportions of metal protective pigment or mixtureof metal pigment with aforesaid metal compound for each part of thecolloidal binder there is achieved film toughness, good filmflexibility, high film hardness and complete film continuity even in avery light single or priming coating which rapidly self-cures to awater-insoluble condition in the film.

Hardness of the lithium hydroxide-silica and metal pigment film may beenhanced in certain cases by suitable selection of suitable hard pigmentand hard extender as fillers. Non-uniformity in film hardness, which isobserved with alkali metal silicate binder, is not encountered with thecoating composition of the invention.

An outstanding and unique characteristic of the lithiumhydroxide-colloidal silica binder of the invention is its quickself-curing characteristic to a state of waterinsolubility in thepresence of finely divided metal pigment. This characteristic makesunnecessary the addition of curing agents or after treatment of thecoating with additional curing agents; in the absence of curing agentsand for ordinary outdoor use, the present binder-metal composition canbe applied to give a single finish coating, pigmented as desired, withexcellent common resisting characteristics for underlying metal. Thissingle coating exhibits good abrasion resistance, exce llefit'resistanceto al SE9.I .d-i1i1m 0f climate and weather,excellentresistance 6 salt water when: sprayto be eminentlysuitable forexposure to the elements at marine installations or for protection inchemical plants wherein only mild chemicals come into contact with thecoating. Completely cured thin films (0.3-3 mils) as well as thick films(6-20 mils) are readily obtained.

In the form of metal pigmented inorganic paint, the film-forming binderbecomes insoluble and resistant to water in a matter of minutes afterapplication i078 metal I I n I a a mw-n-o base; this initial self-curingreaction taking place in a M u u The self-curing action continues for atleast several hours after the initial stage and proceeds up to severalmonths in some cases. The chemistry of the curing reaction is not wellunderstood, but it is believed that the insolubilization of the filmtakes place in two rather distinct stages:

(1) A stage of evaporation to eliminate the aqueous vehicle whichproceeds rapidly.

lithiumoxide-silica ratios of 1:3 to 1:5 unusual hardness flexibilityand toughness are achieved and excellent corrosion protection isobserved, especially for ferrous metal. At these ratios, the filmself-cures even in thin coatings (2) A stage of chemical reaction oflithium hydroxide and ,results an extfemely desirable chfimicauy andcolloidal silica binder with the metal Protecting physically resistantcoating. These characteristics are had ment which Proceeds SW1ythroughout the range of 1:3 to 1:5 as shown in Table I. It is noted thatlithium salts of weak acids, such as L TH UM YROX E-COLLOIDAL SILICABIND- silicic acid are much more insoluble than are the cor- 10 ER ORM OEFFECT OF Li O/SiO responding silicates of sodium and potassium or otherMOLAR RATIO Table 1 Solutions 1 2 3 4 5 6 7 8 9 Colloidal Silica Solids0 0 30 30 30 33 30 30 Water 102 102 102 102 102 102 102 102 LithiumHydroxide-.- 2 4 6 8 9 10 12 14 Molar Ratio Liz0:Si0:- 1:20.8 1:10.4 1:71:5.2 1:4.6 1:4.2 1:3.5 1:3

Increasing ratio of Lii0) linity in the aqueous paint being due to thealkali metal hydroxide used as stabilizer.

In the second stage of chemical reaction by insolubilization, theinorganic binder and pigment ingredients in the film react slowly andcontinue to react slowly over a In the above table, sample 1 is thecontrol. Samples 2-9 were all superior to the control for making waterresistant self-curing films.

Samples 2 and 3 do not harden as rapidly on air drying as samples 49.However, sample 2 represented a distinct improvement in quality of filmover the control sample, sample 1.

For each lithium hydroxide-silica ratio there is a maximum quantity ofmetal pigment such as red lead or zinc dust which can be bound. Withlower concentrations of the lithium alkali this amount of zinc is notalways sufiicient to prevent pinpoint corrosion and these variations aredemonstrated in Tables II and III below.

Outstanding rates of hardening were noted for ratios of 124.6 andhigher.

period of as long as several months to form a complex inter-linked,insoluble film. The lithium silicate ingredient and the metal pigmentsuch as zinc combine with carbonates and silicates, which are present inthe chemically insolubilized film.

In the case of lead being present as the metal pigment, lead reacts withthe lithium hydroxide and colloidal silica ingredients and appears to bepresent in the complex in the form of an added insoluble lead oxide orcomplex salt with silica. That the chemical reaction occurs is indicatednot only by the fact that the film becomes much harder, tougher and moreadherent with time, but also by the fact that the film gradually changesin color after drying, the color changes continuing over long timeperiods.

PROPORTIONS OF LITHIUM HYDROXIDE TO COL- LOIDAL SILICA IN THE COATINGCOMPOSITION Table I below illustrates critical factors in lithiumhydroxide-colloidal silica proportions.

Highly favorable film binding and working properties are obtained withbinder having a molecular ratio of lithium oxide to silica of at leastabout 1:40 and up to 122.5. In the presence of zinc the upper limit ispreferably 1:3.5. In the presence of aluminum the upper limit preferably1:5.0. The coating shown in-Table I below based on samples 3-9 wereoutstanding on steel. At

By critical examination of the films, it was determined that the initialhardness of the films goes up with increasing alkali content while theporosity drops. Although these films are all substantially insolubleafter 24-48 hours and will withstand immersion in neutral aqueoussolutions, water tends to dilfuse along the surfaces at rates governedby the continuity of the coating.

EFFECT OF METAL PIGMENT IN MINIMUM AMOUNT IN ALKALI STABILIZED LITHIUMHYDROXIDE-COLLOIDAL SILICA FORMULA- TION For each lithiumhydroxide-silica ratio there is a maxi mum quantity of metal pigmentwhich can be bound .such as for example, red lead or zinc dust. Withlower concentrations of lithium hydroxide, the amount of zinc may notalways be sufiicient to prevent pinpoint corrosion and spotting. Thus,the amount of lithium hydroxide as well as the amount of metal pigmentare interrelated.

Best results in eliminating the spots and pinpoint corrosion occur whenthe ratio of lithium hydroxide monohydrate to silica is 1:3 or higher(Li O:SiO =1:4.2). At these levels of lithium silicate binder theminimum amount of zinc dust which is needed to eliminate pinpoints ofcorrosion is 450 parts in parts of aqueous binder. The maximum amount ofzinc which can be satisfactorily bound is about 800 parts, and this isdemonstrated in Table III below:

form of dispersed particles having a particle size of 1 to 100millimicrons and having the alkali as sodium hydroxide or potassiumhydroxide substantially all outside of the silica particles.

The colloidal silica used in the foregoing and following examples of thetables is composed of 29 to 31% SiO about 2.5 to about 0.3% Na O or K 0and a maximum of 0.15% sulfates as Na SO and is obtainable commerciallyin the form of a water slurry containing about solids. The alkalistabilizer may be present in amounts up to about 4% as Na O or K 0. Thesilica particles in colloidal silica are extremely small, ranging fromabout 0.01 to 0.03 micron in maximum dimension.

The colloidal silica used herein is described in detail in US. PatentNos. 2,244,325, issued June. 3, 1941; 2,574,902, issued November 13,1951, and 2,597,872, issued May 27, 1952, and may be prepared by passingalkali metal silicate through an ion exchange resin to remove the alkalias described in U.S. Patent No.

for the SiO sol to prevent condensation of the SiO particles.

A number of types of alkali stabilized colloidal silica which may beused are listed as follows:

Trade Name Solids pH Ludox H 30. 0 9. 8 Ludox LS 30. 0 8. 4 Syton C-3030. 0 9. 8 Syton 0-15 15.0 9.8 Nalcoag 1015... 15. 0 8. 6 Naleoag 103030.0 10. 2 Nalcoag 1035-.. 35.0 8. 6 Nalcoag 1050 50.0 9. 0

To colloidal commercial silica sol listed above, a solution of lithiumhydroxide is added and these components react to form a soft gel whichbreaks down to a slightly cloudy solution after agitation and heating.The change in character of the colloidal silica component is seen bycasting thin films on glass plates and comparing them with films fromcolloidal silica sols and low alkaline silicate solutions respectively.As the lithium hydroxide content increases from a control of noaddition, the precipitated particles in dried films go fromdiscontinuous curled platelets (no alkali) to a continuous cohesive film(high alkali). The intermediate concentrations produce films withproperties lying between the two extremes.

If the lithium hydroxide content is too low, the colloidal nature of thesilica will be exaggerated. This is probably due to the lithiumhydroxide diifusing from the silica, leaving the latter in a more rigidstate where shrinkage forces exceed cohesive forces. Small amounts ofalkali in the form of NaOH or KOH may be added to stabilize the lithiumhydroxide colloidal silica in amounts expressed as Na O or K 0 up toabout 4% by weight of the water present.

ZINC PIGMENTED FORMULATIONS FOR ANTI-CORROSION COATINGS The followingTable IV indicates proportions of preferrcd in situ formulations ofcolloidal silica, lithium hydroxide and zinc dust which makesatisfactory self-curing films in accordance with the invention.

Table IV Weight Weight Lithium Li O/SiOz, M01 Weight Silica aterHydrltlaxiidetlviono- Ratio Wefght Zinc Dust y a e .88+8.71 NaOH-.-14.03 (1 =.121 450-800 (typical LiOH-Hg0 mixed silicate formula).

+.879 NaOH).

2,244,325. If all of the alkali is removed from the silicate, theresulting sols are not stable, but they can be stabilized by adding asmall amount of alkali such as N320 or K 0.

The silica-alkali ratio varies in commercially available colloidalsilica from about :1 to about 130:1, the dispersion containing discretesilica particles, having a molecular weight, as determined by lightscattering of more than one-half million. It has a relative viscosity,at 10% SiO from 1.15 to 1.55 and generally contains from 20 to 35% byweight of SiO:.

The alkali present in colloidal silica is not uniformly distributedthroughout the Si0 particles as it is in conventional silicate such aswater glass but is substantially The formulae below the line in Table IVhardened much more rapidly than those above the line.

The formulations summarized in foregoing Table IV utilize zinc dusthaving the following particle size disribu ion: w

The particle size distribution of the zinc dust is such all outside theSiO particles and acts as a stabilizer as to substantially eliminatelarge particles above 20 9 microns in size since large particles presentin substantial proportion more than 10%, causes cracking of the film ondrying.

In the formulation shown in the above table, when the mol ratio oflithium oxide to silica goes beyond 1:35, the mixture of binder and zincwill not harden rapidly enough on air drying to meet the strictrequirements for self-curing rapid hardening anti-corrosion coatmgs.

Graphite may be incorporated into the paint at a concentration of 1% byweight of the zinc dust which is present without degrading, in anymaterial respect, the film properties which are achieved. Its inclusionappears to enhance the conductivity of the zinc-filled coating.

DECORATIVE PIGMENTS IN LITHIUM HYDROXIDE-COLLOIDAL SILICA COATINGS Theinvention includes pigmented lithium hydroxidecolloidal silica paintsfree from protective metal pigment and pigmented with bright inorganicpigments as illustrated in Table V below.

Highly useful lithium hydroxide-colloidal silica paints pigmented withcommon decorative pigments can be made in accordance with the typicalformulations set out in Table V below. Because the usual pigments are ofuniform and ultrafine particle size, relatively coarse Red lead can beused to replace up to about 16% of filler may be added to avoid crackingof the film.

Table V Weight Wt Color Pigment Weight Weight g f ag Liio sioi. Wt.0-1000 sioi Water Mole Ratio Pig. lyr a hydrate Green (Chromic Oxidegreen pigment). 190 8.97 1 4. 67 175 85 Red (Oxide Red Mapico 297) 30190 8.97 1 4. 67 175 White (Titanox A-WD) 30 234 8.97 1 4. 67 42 ZincOxide (Horsehead XX-503 grade) 46 Zinc Oxide (Kadox 15 grade) 11 thezinc with substantially no significant loss of cathodic metal. Oneadvantage of adding red lead is its inhibition of hydrogen gas formationand another advantage is the factor of lower filler cost.

Inert fillers such as mica and barytes can also be substituted in partfor the zinc, but must be used in limited proportions since excessiveamounts tend to detract from the anodic properties of the zincprotective agent. In amounts up to 15% of the zinc the filler can beused to decrease tendency toward cracking when the zinc dust does notcontainenough large particlesin the 3-12 micron range and the inertfillers also provide lower raw material cost.

The fillgs which by testing show the best advantages are very fine 1%,known as grade C-1000 mica and coarse barytes.

In the foregoing Table IV, the compositions have been illustrated withzinc dust, with mixtures of zinc and red lead, mixtures of zinc andgraphite, mixtures of zinc and mica, and mixtures of zinc and barytes.

Following is a description of aluminum lithium silicate compositionwhich were tested in Florida marine atmosphere and found to be verysatisfactory. Examination of the filmafter one weeks exposure showedthat it was insoluble and reasonably hard. A faint white efilorescencewas noted by close inspection of the surface, but it could not be seenat a distance beyond several feet. This coating composition providedoutstanding protection for the exteriors of storage tanks, particularlyin coastal locations where the tanks were exposed to salt air from thenearby ocean.

ALUMINUM PIGMENT FORMULATION FOR OUTDOOR EXPOSURE The aluminum powderused here is Meta'ls Disintegrating Companys MD 5100 which is defined inFed. Spec.

In the foregoing Tables I-V and other examples, the metal protectingpigment is exemplified with zinc dust and with aluminum flake. There mayalso be used powdered lead having a particle size varying from about 1micron to about 15 microns, preferably an average size of about 8-10microns, the proportion of powdered lead to binder varying from about 20to about 40 parts per part of colloidal lithium silicate.

When lead is used, as in the case with zinc pigment such finely dividedpigment and extender compounds as lead oxide, lead dioxide, leadchromate, zinc oxide, iron oxide and aluminum oxide may be added inamounts up to 15% of the lead to replace part of the lead. Here thebenefit of additional protection or inhibition against hydrogen gasformation is supplied in the paint containing the compounds of lead,e.g., lead oxide, red lead, LapP e at. 2 Q 9 -2e i9 s-. i Jangeithefinely divided extender, the blgnfidii g f tlggetal pigment 1 eindefirlajbii rmgyg 0 illustrate, either zinc or aluminum dust havm aarticle sizgxange W081i be blended with inert extender such as mica, 1amum dioxide, zinc oxide, chromic oxide, alumina, barytes,iron oxide,etc. in a coarser particle range of from about 6-40 microns. Since theextender has a substantial fraction of coarser particles above 12.microns in diameter, it helps distribute the finer metal particles. Thecoarser extender may be used to advantage with finer particle sizeprotective metal pigment to enhance the covering characteristics andflexibility of the metal pigment.

Since the binder, metal pigment, auxiliary anti-corrosion metal oxide,fiexibilizers such as mica or barytes and extender of the presentcoating arm wholly inorganic, the film is unaffected by organic solventsor by liquid water after the film is cured. Thus, the film is alsowholly unaffected by ultraviolet light, by changes in humidity, byneutral inorganic salts, by water vapor and by organic solvents. It isaccordingly, very useful in providing resistance against organicchemicals such as petroleum hydrocarbons, toluol, ether, xylol, ketones,chlorinated hydrocarbons, etc.

If softening of the film occurs in a freshly applied coating due tocontact or immersion in water, this softening characteristic quicklydisappears after the coating has been completely cured by standing.

By using water resistant filler, the softening can be diminished.However, it is desirable that the coatings 11 first be set in asubstantially completely water-resistant state.

If conditions of humidity or moisture are such that the normalself-curing action of the present coating composition is not rapidenough to suit the requirements which are to be met, or if watersensitive filler is employed which delays self-curing beyond that whichcan be tolerated, acid salts or dilute mineral or organic acids may beused in aqueous solution as curing agents which appear to accelerate theself-curing mechanism of the binder of the invention. Illustrativeexamples of such acids are dilute (e.g., 20%) solutions of sulfuricacid, hydrochloric acid, acetic acid, phosphoric acid, nitric acid or{acid salts of strong mineral acids such as sodium bisulfate, sodiumdihydrogen phosphate. However, it is ob- ;served that for most cases,the use of this type of acid main advantage of the binder in the coatingof the invention.

The fact that the highly anti-corrosive protection is provided in but asingle coating acting as the finished coating and having very valuableproperties of abrasion resistance, hardness, chemical resistance,weathering resistance in this single continuously covered finishcoating, even in thin films, provides an important commercial advantageleading to commercial preference for the coating of the invention ascompared to other inorganic coatings which are commercially available.

We claim:

1. An inorganic protective coating composition for metal surfaces whichis self-curing at room temperature consisting essentially of water as adispersing medium and a film-forming binder consisting of lithiumhydroxide and alkali stabi ed co oidal silica having-a particle size ofabout 1 to about 100 millimicrons, the molar ratio of lithium hydroxideexpressed as lithium oxide to colloidal silica, on a dry weight basis,being about 1:40 up to about 1:2.5 and from about 1 part up to about 25parts of a finely divided metal pigment per part of said binder, saidpigment being reactive with said binder and selected from the groupconsisting of zinc dust, 'powdered. lead and aluminum flake, there beingpresent from about 0.3% to about 4% by weight of alkali as Na 0 and K 0in said stabilized colloidal silica, the proportion by weight of saidreactive pigment to said film-forming binder being at least 1:1 for thelightest density reactive pigment and varying up to a maximum of about25: 1.

2. An inorganic protective coating composition for a metal surfaceconsisting essentially of a film-forming binder consisting of lithiumhydroxide and alkali stabilized colloidal silica having a particle sizeof about 1 to about 100 millimicrons, the molar ratio of lithiumhydroxide expressed as lithium oxide to colloidal silica, on a dryweight basis, being about 1:40 up to about 1:2.5 and at least about 1part of a finely divided metal pigment per part of said binder, saidpigment being less than 15 microns in size and being reactive with saidbinder and selected from the group consisting of zinc dust, powderedlead and aluminum flake, the proportion by weight of said reactivepigment to said film-forming binder being at least about 1:1 for thelightest density reactive pigment and varying up to 25:1 for heavierdensity reactive pigment.

3. A coating composition as claimed in claim 1 incorporating adecorative pigment consisting of an oxide se- 12 lected from the groupconsisting of chromic oxide, iron oxide, zinc oxide and lead oxide. M

4. A coating composition as claimed in claim 2 wherein said reactivemetal pigment is zinc.

5. A coating composition as claimed in claim 2 wherein said reactivemetal pigment is aluminum.

6. A coating composition as claimed in claim 2 wherein said reactivemetal pigment is lead.

7. A coating composition as claimed in claim 2 wherein said molar ratioof lithium oxide to silica is from 1:40 to 1:3 and said reactive metalis zinc.

8. A coating composition as claTm'd in claim 2 wherein there is presentan extender in an amount up to 15% of said reactive metal pigment.

9. A coating composition as claimed in claim 2 wherein there is alsopresent a finely divided coloring pigment having a coarser particle sizethan said metal pigment in the range of from 6 microns to 40 micronsparticle size.

10. A decorative coating composition as claimed in claim 9 wherein themolar ratio of Li O to SiO is about 1 to 4.67 and in which mica is alsopresent.

11. A coating composition as claimed in claim 9 wherein carbon black isthe coloring pigment which is present.

12. A coating composition as claimed in claim 9 wherein said coarsercoloring pigment includes barytes.

13. A coating composition as claimed in claim 9 wherein graphite is thecoloring pigment which is present.

14. A coating composition as claimed in claim 9 wherein coloring pigmentis titanium dioxide.

15. A composition afilifified in claim 9 including a gag as an extender.

16. A method of making an aqueous inorganic, selfcuring coatingcomposition adapted to protect metal surfaces against corrosionconsisting essentially of adding lithium hydroxide solution to alkalistabilized colloidal silica of a particle size of about 1 to about 100millimicrons, said colloidal silica being stabilized with from about 0.3to about 4.0% of alkali metal hydroxide selected from the groupconsisting of sodium hydroxide and potassium hydroxide, these expressedas Na O and K 0, respectively, by weight of the water present in themixture, adjusting the molar ratio of lithium hydroxide expressed as LiO to colloidal silica to a value lying between about 1:40 up to 1:2.5,and mixing to form a selfcuring binder and adding a reactive metalpigment to said self-curing binder whereby initial hardening of thebinder at room temperature occurs mainly by evaporation on drying andsaid hardening continues after drying at room temperature by interactionof the binder ingredients and said reactive metal, said reactive pigmentbeing selected from the group consisting of zinc dust, powdered lead andaluminum flake, the proportion by weight of said reactive pigment tosaid film-forming binder being at least 1:1 for the lightest densityreactive pigments and extending up to a maximum of about 25:1.

References Cited in the file of this patent UNITED STATES PATENTSNightingall et al May 4, 1948 McDonald Nov. 27, 1951 Drummond Oct. 2,1956 Seidl Apr. 4, 1961 Munger Aug. 29, 1961 OTHER REFERENCES

1. AN INORGANIC PROTECTIVE COATING COMPOSITION FOR METAL SURFACES WHICHIS SELF-CURING AT ROOM TEMPERATURE CONSISTING ESSENTIALLY OF WATER AS ADISPERSING MEDIUM AND A FILM-FORMING BINDER CONSISTING OF LITHIUMHYDROXIDE AND ALKALI STABILIZED COLLOIDAL SILICA HAVING A PARTICLE SIZEOF ABOUT 1 TO ABOUT 100 MILLIMICRONS, THE MOLAR RATIO OF LITHIUMHYDROXIDE EXPRESSED AS LITHIUM OXIDE TO COLLOIDAL SILICA, ON A DRYWEIGHT BASIS, BEING ABOUT 1:40 UP TO ABOUT 1:2.5 AND FROM ABOUT 1 PARTUP TO ABOUT 25 PARTS OF A FINELY DIVIDED METAL PIGMENT PER PART OF SAIDBINDER, SAID PIGMENT BEING REACTIVE WITH SAID BINDER AND SELECTED FROMTHE GROUP CONSISTING OF ZINC DUST, POWDERED LEAD AND ALUMINUM FLAKE,THERE BEING PRESENT FROM ABOUT 0.3% TO ABOUT 4% BY WEIGHT OF ALKALI ASNA2O AND K2O IN SAID STABILIZED COLLOIDAL SILICA, THE PROPORTION BYWEIGHT OF SAID REACTIVE PIGMENT TO SAID FILM-FORMING BINDER BEING ATLEAST 1:1 FOR THE LIGHTEST DENSITY REACTIVE PIGMENT AND VARYING UP TO AMAXIMUM OF ABOUT 25:1.